Movieverse is the canon or fanon (depending on one's point of view) created by a film adaptation of a book. It isn't uncommon for the movieverse version of events to become more common in fanfiction than the actual version. Even in stories that mainly follow the books, details from the movies can slip in, like Lucius Malfoy carrying a cane or Faramir having reddish-brown hair.
Badfic is very frequently movieverse. This is probably because someone who doesn't like the fandom enough to actually read the Glaurunging books isn't very likely to write decent fanfic. Also, movies never provide as much information about a world as the books, which means that they will likely get at least some stuff wrong. This is far worse when the movies encourage actual misconceptions—this is especially a problem in the Tolkien Fandom. The problem can be summarized like this: a fanfic will always be at least a slightly skewed vision of the fandom, and so will the movie adaptation. If you have a fanfic based on a movie, then it'll end up twice as skewed. (Or skewed back in the right direction, but that never happens.)
In rare cases, movieverse canonicity becomes even fuzzier. The Hitchhiker's Guide to the Galaxy, for example, has had several slightly different incarnations, including the book, tv mini-series, radio drama, and film, all of which were created by Douglas Adams. In cases of multiple versions, variations in canon can be expected and are acceptable in fic.